The present invention relates to refining discs and plate segments for refining discs, and more particularly to the shape of the bars and grooves that define the refining elements of the discs or segments. The plate segments may be used, for example, in refining machines for disperging, deflaking, and for refining all ranges of consistency (HiCo, LoCo and MC) of lignocellulosic material. Further, the invention may be applied to various refiner geometries, such as disc refiners, conical refiners, double disc refiners, double conical refiners, cylindrical refiners, and double cylindrical refiners.
Lignocellulosic material, such as wood chips, saw dust and other wood or plant fibrous material, is refined by mechanical refiners that separate fibers from the network of fibers that form the material. Disc refiners for lignocellulosic material are fitted with refining discs or disc segments that are arranged to form a disc. The discs are also referred to as “plates.” The refiner positions two opposing discs, such that one disc rotates relative to the other disc. The fibrous material to be refined flows through a center inlet of one of the discs and into a gap between the two refining discs. As one or both of the discs rotate, centrifugal forces move the material radially outward through the gap and out the radial periphery of the disc.
The opposing surfaces of the discs include annular sections having bars and grooves. The grooves provide passages through which material moves in a radial plane between the surfaces of the disc. The material also moves out of the radial plane from the grooves and over the bars. As the material moves over the bars, the material enters a refining gap between crossing bars of the opposing discs. The crossing of bars apply forces to the material in the refining gap that act to separate the fibers in the material and to cause plastic deformation in the walls of said fibers. The repeated application of forces in the refining gap refines the material into a pulp of separated and refined fibers.
As the leading edges of the bars cross, the material is “stapled” between the bars. Stapling refers to the forces applied by the leading faces and edges of opposite crossing bars to the fibrous material as the leading faces and edges overlap. As the bars cross on opposite discs cross, there is an instantaneous overlap between the leading faces of the crossing bars. This overlap forms an instantaneous crossing angle which has a vital influence on the material stapling and/or the covering capability of the leading edges of the bars.
FIG. 1 shows in cross-section a few bars 10 and grooves 12 of a conventional high performance low consistency refiner plate 14. These bars 10 typically feature a high bar height to bar width ratio and have a zero or nearly zero degree draft angle. The draft angle is the angle between the leading or trailing face (sidewall) 16 of a bar and a line 18 parallel to an axis of the plate. The refiner plate 14 may be formed of a single alloy, such as from the 17-4PH stainless steel alloy group. Refiner plates formed of the 17-4PH alloy tend to have a bar height to bar width ratios that are larger than refiner plates formed of other metal alloys. These large ratios result in narrow bars and sharp corners at the roots of the bars. Plates formed of the 17-4PH alloy tend to have high strength and bars that are not prone to failure.
The zero degree draft angle, narrow bars and deep grooves of conventional high performance plates may result in excessive and unsustainable stresses at the root 20 of the bars. Bar failure, e.g., shearing of bars at the root, may result, especially if the plate is formed of materials other than from the 17-4PH alloy group. Plates formed of the high strength 17-4PH alloy tend to have excessive wear and short operational lives when subjected to an abrasive refining environment. Refiner plates formed of alloys other than 17-4PH tend to have bar and groove pattern designs constrained by the brittleness of the utilized alloy material.
Because of excessive stresses on high and narrow bars, plates having conventional high performance bar and groove patterns may not be practically formed of high wear resistance stainless steel material. Stainless steel with good wear characteristics has been used to form less demanding refiner plate designs. But unsuccessful attempts have been made to develop alloys combining the toughness of the 17-4PH alloy with the wear resistance of other stainless steel alloys. Despite the efforts to find or develop suitable alloys, high performance refiner plate patterns keep break when formed of materials (other than 17-4PH) having inadequate energy absorption potential.
FIG. 2 is a cross-sectional diagram of another conventional high performance low consistency refiner plate 22. The cross-section shows the bars 24 and grooves 26 of the plate 22. The draft angle 28 is, for example, five (5) degrees which is considered a large draft angle. Large draft angles result in bars formed of greater amounts of material than bars with shallow draft angles, e.g., draft angles less than five degrees. The greater amount of material resides in the wide base of the bars.
The greater amount of bar material in bars with large draft angles increases the moment of inertia of the bars. The added bar material and greater inertia enhances the breakage resistance of the bars. The wide draft angle also lowers the applicable bar height to bar width ratio and thus leads to lower bar edge length potential. The consequences of lower bar height to width ratios and lower edge lengths are typically: lower energy efficiency, suboptimal fiber quality development, and a reduction in hydraulic capacity due to the non-linear reduction in open area in the grooves in the course of the plate's service life caused by large draft angles. Large draft angles also reduce the “sharpness” of the leading edges of the bars which may have a negative impact on the quality consistency over the service life of the plates.
There is a long felt need for high performance refiner plates and techniques to design plates that may be formed of a wide range of metal alloys, e.g., other than the 17-4PH alloy, that are now typically used to form conventional plates only. Further, there is a long felt need for refiner plates that provide both the refining characteristics typically found only with high performance refiner plates and have a long service life through enhanced wear resistance.